I would assume that points cards will just be converted when you use them and will then be gradually phased out and replaced with eShop cards.

I doubt it. It's too complex for Nintendo to additionally add a small, for most meaningless operation when they are extremely busy elsewhere with things that could be a lot more useful, doing this would be counter-productive for Nintendo, in a sense of time wasted, rather than money.

I've heard people saying they were able to buy the eShop cards at BestBuy, but nowhere else. It would be nice if we could still use the Nintendo points cards on the eShop, but there's probably one major reason they're not carrying them forward: retailers that charge $25 for a 2000-point card. Imagine the complaints if people paid $25, then loaded the card and noticed they were only credited with $20. At least cash cards will stop any retailers from overcharging for the cards now.